1. Field of the Invention
The present invention relates to a light emitting diode (LED) chip, and more particularly to an LED chip characterized by satisfactory light emitting efficiency.
2. Description of Related Art
With a progress in semiconductor technologies, an LED now has advantages of high luminance, low power consumption, compactness, low driving voltage, mercury free, and so forth. Therefore, the LED has been extensively applied in the field of displays and illumination. Generally, light emitting efficiency of the LED is mainly determined upon internal quantum efficiency and external quantum efficiency (light extraction efficiency) of an active layer in the LED. Epitaxial quality of a semiconductor layer and a method of stacking film layers for forming the semiconductor layer contribute to the favorable internal quantum efficiency, while the enhancement of the external quantum efficiency is achieved when light emitted by an LED chip is effectively propagated. Namely, the external quantum efficiency is associated with external optical designs of the LED chip.
FIG. 1A is a top view of a conventional LED chip, and FIG. 1B is a schematic cross-sectional view taken along a section line I-I depicted in FIG. 1A. Referring to FIGS. 1A and 1B, a conventional LED chip 100 includes a substrate 110, a semiconductor device layer 120, an N-type electrode 130a, and a P-type electrode 130b. The semiconductor device layer 120 is disposed on the substrate 110 and includes an N-type semiconductor layer 122, an active layer 124, and a P-type semiconductor layer 126. The N-type semiconductor layer 122 is disposed on the substrate 110. The active layer 124 is disposed on a portion of the N-type semiconductor layer 122. The P-type semiconductor layer 126 is disposed on the active layer 124. Besides, the N-type electrode 130a is disposed on the N-type semiconductor layer 122 and has a favorable ohmic contact with the N-type semiconductor layer 122. By contrast, the P-type electrode 130b is disposed on the P-type semiconductor layer 126 and has a favorable ohmic contact with the P-type semiconductor layer 126.
It can be observed from FIGS. 1A and 1B that light L′ emitted from the active layer 124 of the LED chip 100 is transmitted toward various directions. Nonetheless, when the LED chip 100 is applied to the filed of displays and illumination, the light emitted by the LED chip 100 is often concentrated within a certain range, so as to provide a highly directional LED in which an angle of dispersion is relatively small. Therefore, how to utilize the light transmitted toward different directions in an effective manner has become an essential topic.